The invention relates to a microfilm reader provided with a holding frame for holding a microfiche. The holding frame is guided for movement along two perpendicular coordinate axes, so that selected portions of the microfiche can be moved into position for projection.
With known devices of this general type, the holding frame for the microfiche is shifted along the two perpendicular coordinate axes, within the plane defined by such axes, by hand, until such time as the desired one of the microfilm images, from out the plurality of rows and columns of images on the microfiche, has been brought into the projection position of the viewing objective.
Because such shifting of the holding frame for the microfiche should be easily performed manually, it has been desired to provide for the microfiche holding frame a mounting means which exhibits the least possible frictional resistance to such shifting movement and which permits very light manually exerted shifting forces to effect the required shifting movement. However, when such expedients are adopted, there arises the danger of unintentional shifting of the microfiche holding frame due to light accidental touching of the frame, or due to movement of the supporting structure for the microfilm reader, for example if the microfilm reader is located in a moving vehicle. Such unintentional shifting results in the display of portions of the microfiche other than the portion desired to be projected.
With these difficulties in mind, it has been proposed to provide an arresting means for arresting the microfiche holding frame in the selected position. The proposed arresting means made use of braking elements operable for engaging brake bars to arrest the holding frame in place. The braking elements were activatable by means of control cables. This arresting mechanism was of relatively complicated construction and moreover exhibited the significant disadvantage that, in the process of applying the requisite arresting force, torques and lever forces were transmitted to the holding frame, and these turning forces had the effect of tending again to displace the microfiche holding frame from the selected position.